The present invention relates to synthetic polymers, and more particularly, to synthetic polymers having high molecular weights and low polydispersities and methods of synthesis and use thereof.
Polymers have extraordinary ranges of properties that make them useful for a number of applications. For example, polymers can be used as large scale structural materials (e.g., carbon fiber reinforced thermoset polymers used in airplanes) as well as high value-added ingredients on the scale of grams (e.g., lithography, drug delivery, etc.). Polymers are also widely used in everyday applications. Some of these uses include, but are not limited to, disposable packaging, paper, film, tubing, and the like.
The versatility of polymers, in particular synthetic polymers, may be a direct result of the versatility in their physical properties, which in turn reflects the advances in molecular synthesis and design over the years. In other words, synthetic polymers have numerous tunable properties that allow for their expanding use across various technology platforms. These tunable properties may be optimally tapped once the polymer is sufficiently large and/or have a sufficiently narrow distribution of molecular mass.
Currently, some of the main desired goals of synthetic polymer chemistry include achieving high degrees of control over polymer architecture like polymer length, creation of well-defined diblock (or polyblock) copolymers, creation of specifically branched structures (e.g., star polymers, dendritic polymers, brush polymers, and self-assembled polymers), and design of these systems at increasingly large sizes. For example, it is believed that current methods are limited to the synthesis of block copolymers having individual block sizes between about 10 kDa and about 100 kDa in molecular weight.
While there are many different types of synthetic polymers, synthesis generally involves a polymerization reaction in which monomer molecules react to form a multi-dimensional network of polymer chains. Examples of known polymerization reactions include ring-opening polymerization, addition polymerization (e.g., living polymerization), condensation, etc.
One of the main limitations of current polymerization techniques is that individual polymer chains rarely have the same degree of polymerization and molar mass. In other words, polymerization reactions typically generate a distribution of polymer sizes around an average value. In some cases, this heterogeneity is undesirable. For example, the performance of even relatively small polymers such as photoresists and polyacrylate detergents (˜5000 MW) goes up as the polydispersity index (PDI) goes down. It is also generally believed that increasing polymer chain length improves many physical properties, notably the mechanical characteristics. However, the technical challenges of synthesizing low PDI polymers typically become more pronounced as one attempts to synthesize larger polymers.
As used herein, “polydispersity index” refers to a measure of the distribution of molecular mass in a given polymer sample. The polydispersity index is calculated by dividing the weight average molecular weight (Mw) by the number average molecular weight (Mn). As used herein, the term “weight average molecular weight” generally refers to a molecular weight measurement that depends on the contributions of polymer molecules according to their sizes. As used herein, the term “number average molecular weight” generally refers to a molecular weight measurement that is calculated by dividing the total weight of all the polymer molecules in a sample with the total number of polymer molecules in the sample. These terms are well-known by those of ordinary skill in the art.
Conventional polymerization methods typically cannot produce or have trouble producing high molecular weight (those above about 500,000) polymers that have low PDIs (typically less than about 1.25). Polymers with molecular weights that exceed 106 g/mol are typically referred to as ultra-high molecular weight (UHMW) polymers. Examples of UHMW that have been synthesized include, but are not limited to, polyethylene, polypropylene, polyisobutylene, polyacrylamide, polyisoprene, poly(ethyleneoxide), polystyrene, poly(vinylacetate), and the like.
Living polymerization is a technique that is often used to synthesize polymers that have low PDIs. This technique employs the use of living polymerization systems, such as organometallic systems in atom transfer radical polymerization (ATRP), nitrogen-mediated polymerization (NMP), or chain transfer agents (e.g., thiols, halocarbons, etc.) in reversible addition-fragmentation chain transfer (RAFT), in order to achieve narrow polymer length dispersities. A living polymerization reaction generally involves forming terminal groups that can be used to polymerize other monomers. FIG. 1 (bottom) shows an example of a living polymerization reaction, in which each polymerization step is protected. FIG. 1 (top) also shows an example of a conventional free-radical polymerization reaction, in which unprotected radicals are generated after each polymerization step. Thus, one of the characteristics of living polymerization is that the ability of a growing polymer chain to terminate has been removed.
Polydispersity index can be used to characterize or describe the effectiveness of a living polymerization reaction. For example, a PDI of 1 indicates a highly homogeneous reaction in which each polymer chain is identical in length. Generally, PDI values increase as a reaction product becomes more heterogeneous. Certain reactions such as free radical polymerization are often difficult to control and form polymers that have a wide range of sizes. A PDI of 1.50 represents the “ideal” free radical polymerization that is performed in infinitely dilute conditions. These reactions usually occur at lower conversion rates.
Currently, living polymerization techniques are usually limited to polymers that are around 200 kDa and PDIs typically on the order of 1.5 or greater. Yet another drawback of living polymerization is that the technique is typically performed in organic solvents using environmentally caustic reagents that are difficult to remove.